The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions.
Historically, early hip replacement consisted of a non-modular femoral head with a single neck option, the so-called “monobloc”. This meant that restoring the leg length and offset was difficult, and may have resulted in instability and abductor dysfunction. As a result, modularity was introduced into the design of hip prosthesis and has become increasingly common in the last two decades. Modularity can be exhibited at the junction between the head and the neck, and at the junction between the trunnion (neck) and the body. The neck head junction typically consists of a trunnion, which has a machined taper allowing for an interference fit. The taper interface is where the femoral head (female taper surface) attaches to the trunnion (male taper) of the femoral stem. This optionality is extremely attractive to the surgeon allowing the ability to more accurately restore leg length, offset, and produce good stability independent of femoral stem fixation. Taper corrosion however, has recently become a clinical issue.
Trunnionosis is defined as wear of the femoral head-neck interface and has been acknowledged as a source of total hip arthroplasty (THA) failure. This phenomena appears to have gained prevalence with newer THA implant designs, particularly when modularity was introduced.
Modularity allows for better intraoperative restoration of leg length and control of hip offset, but while this enables a more customized fit for the patient, it may have untoward effects. This modularity at times may play a role in increased wear and mechanical insufficiency at the trunnion, ultimately leading to revision. By some estimates, trunnionosis accounts for up to 3% of all revision procedures. The exact cause of trunnionosis, which is likely multifactorial, currently remains poorly understood. It is postulated that contributing factors include wear between metal on metal modular junctions, corrosion and fretting damage, and release of metal ions from affected components. Additionally different implant designs and geometries have demonstrated a predisposition to trunnion failure. Although the exact cause of the recent increase of corrosion related complications is unknown, some have hypothesized that having differing alloys at the modular junction, which are under high loads, may lead to increased corrosion and fretting, i.e.: use of cobalt chrome femoral head (CoCr) on Titanium trunnion.
What is needed is a system and method for improving mechanical assemblies, such as prosthetic implants, intended to be installed in living tissue such as bone.